> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 1 Abstract—We present an optical Nyquist-filtering (de)multiplexer using a ring resonator assisted interferometer circuit. It features a near-rectangular passband shape, scalable port count, and can be designed with sub-GHz spectral resolution. The circuit can be constructed using ordinary passive photonic integrated circuit building blocks. In contrast to its counterparts using tapped-delay-line circuit topologies, this requires two- orders-of-magnitude smaller chip area and easier control. The results of this work show the potential for realizing high-spectral- efficiency multi-carrier transceivers and reconfigurable optical add-drop multiplexers in a fully-integrated form. Index Terms—Nyquist filter, multiplexing, WDM, optical signal processing, photonic integrated circuit, waveguide device, discrete Fourier transform, fiber communications, transceiver. I. INTRODUCTION ULTI-CARRIER techniques, such as super-channels and Nyquist wavelength division multiplexing (N-WDM) are of great interest for next-generation high-capacity, elastic optical communication networks [1, 2], as they support high spectrum utilization by allowing carriers to be spaced at or close to signal baud rate. N-WDM is in particular promising for flexible channel management as it allows Nyquist channel shaping and (de)multiplexing functions to be implemented by means of optical filters [3‒5], which reduces the power- consuming workload of digital signal processing in the system and avoids the associated processing latency as well as signal conversions between the optical and electrical domain. For multi-carrier transceiver applications, optical (de)multiplexers that are suitable for N-WDM systems and are able to shape the transmitted spectra to be only slightly wider than the signal baud rate, namely Nyquist-filtering (de)multiplexers, are a highly desired function. Combining Nyquist filtering and (de)multiplexing in one device not only brings benefits to transceiver complexity and overall loss, but also provides a useful building block for constructing reconfigurable optical add-drop multiplexers (ROADMs) [6‒8] for N-WDM-based networks that have great application potential for the future optical communication technologies. To date, a number of studies of Nyquist-filtering (de)multiplexers have been reported. Conventional implementations using free-space optics have demonstrated Issue No xxxxxxxxxxxxxx 2015. This work was supported in part by the Australian Research Council Laureate fellowship with grant no. FL13010041. Dr. Leimeng Zhuang, Dr. Chen Zhu, Yiwei Xie, Dr. Bill Corcoran, and Prof. Arthur J. Lowery is with the Electro-Photonics Laboratory, Department of Electrical and Computer Systems Engineering, Monash University, Australia (Leimeng.zhuang@monash.edu). B. Corcoran and A. J. Lowery are also with great flexibility regarding filter passband characteristics; however, these have a common trade-off between spectral resolution and device complexity [3‒5]. Alternatively, photonic integrated circuit (PIC)-based implementations offer several desirable features, i.e. significant reduction of device size, potential for low-cost fabrication, and simple packaging processes [9‒12]. In terms of the circuit configuration, an arrayed-waveguide grating (AWG) with a synchronized Mach- Zehnder interferometer (MZI) is a widely investigated (de)multiplexer featuring a flat-top passband [13, 14]. However, due to the intrinsic diffraction in the slab waveguide [15], the outer ports of the (de)multiplexer suffer from higher loss and frequency deviation relative to the inner ports. An effective solution for this issue is to replace the AWG by a discrete Fourier transform (DFT) circuit based on a matrix of couplers, phase shifts, and delay lines [16]. When a DFT circuit is combined with additional processing stages, a Nyquist- filtering (de)multiplexer can be devised with custom passband shaping according to various application requirements [17, 18]. Although these designs have been successfully demonstrated, all the (de)multiplexer demonstrations to date are based on tapped-delay-line circuit topologies (characterized by a finite impulse response (FIR) in terms of signal processing), which have, in general, a trade-off between filtering performance and circuit complexity. For the purpose of Nyquist-filtering, such circuit topologies need a significant number of delay lines with a maximum length of >10× the unit delay (which determines the circuit free spectral range (FSR) [19]) to achieve sharp passband transition and sufficient stopband extinction. However, a near-rectangular passband accompanied by a stopband extinction > 35 dB, is highly desired to minimize the inter-sub-carrier/inter-channel crosstalk, which is a key performance metric that determines the practicality of the device. For example, a circuit implementing a raised cosine filter with an excess bandwidth of 2% of the ideal Nyquist bandwidth will require more than a hundred taps, implying an impractical number of delay lines with a maximum delay equal to the product of the tap number and the unit delay [20‒22]. In terms of device fabrication, such a complex circuit requires a large chip area and therefore bears high risk of waveguide quality non-uniformity across the chip and high sensitivity for circuit parameter errors due to fabrication tolerance, which may lead to severe performance degradation and low device yield. the Centre for Ultrahigh-bandwidth Devices for Optical Systems (CUDOS), Australia. Dr. Maurizio Burla is with the Instritut National de la Recherche Scientifique (INRS-EMT), Montréal, Canada. Dr. Chris G. H. Roeloffzen and Mr. Marcel Hoekman are with the SATRAX B.V. and LioniX B.V., The Netherlands. Leimeng Zhuang, Chen Zhu, Yiwei Xie, Maurizio Burla, Chris G. H. Roeloffzen, Marcel Hoekman, Bill Corcoran and Arthur J. Lowery Nyquist-filtering (De)multiplexer Using a Ring Resonator Assisted Interferometer Circuit M